East Antarctic ice basin 'may be at risk'

The East Antarctic ice sheet is thought by most scientists to be stable, reports Alex Kirby. But a German team says it has found how a large part of it could in time melt unstoppably, causing a long term 4 metre rise in global sea levels.

But once it had started, the discharge of ice would slowly but relentlessly continue until the whole basin was empty, even if the climate ceased to warm.

Part of the East Antarctic ice sheet may be less stable than anyone had realised, researchers based in Germany have found.

Writing in Nature Climate Change, two scientists from the Potsdam Institute for Climate Impact Research (PIK) say the melting of quite a small volume of ice on the East Antarctic shore could ultimately trigger a discharge of ice into the ocean which would result in unstoppable sea-level rise for thousands of years ahead.

Their findings, which they say amount to the discovery of a hitherto overlooked source of sea level rise, appear unlikely to happen any time soon. They are based on computer simulations of the Antarctic ice flow using improved data of the ground profile beneath the ice sheet.

'Once uncorked, it empties out'

"East Antarctica's Wilkes Basin is like a bottle on a slant", said Matthias Mengel, the lead author of the study. "Once uncorked, it empties out." The basin is the largest region of marine ice on rocky ground in East Antarctica.

At the moment a rim of ice at the coast holds the ice behind it in place, like a cork holding back the contents of a bottle. The air over Antarctica remains cold, but oceanic warming can cause the ice on the coast to melt. This could make the relatively small 'cork' disappear.

Once it had gone, the result would be a long-term sea level rise of three to four metres. "The full sea-level rise would ultimately be up to 80 times bigger than the initial melting of the ice cork", says the study's co-author, Anders Levermann.

"Until recently, only West Antarctica was considered unstable, but now we know that its ten times bigger counterpart in the East might also be at risk."

Stability 'over-estimated'

Levermann is a lead author of the sea-level change chapter of the most recent scientific assessment report by the Intergovernmental Panel on Climate Change, the IPCC. Its report, published in September 2013, says Antarctica's total sea level contribution could be up to 16 centimetres during this century.

"If half of that ice loss occurred in the ice-cork region, then the discharge would begin. We have probably overestimated the stability of East Antarctica so far", says Levermann.

Melting would cause the retreat of the grounding line, where the ice on the solid continental landmass meets the sea and starts to float. The rocky ground beneath the ice in the Wilkes Basin forms a huge valley below sea-level which slopes downwards as it heads inland.

When the grounding line retreats from its current position on a ridge into the valley, the rim of the ice facing the ocean becomes higher than before. More ice is then pushed into the sea, eventually breaking off and melting. And the warmer it gets, the faster this happens.

For all the ice in the Wilkes Basin to be lost in this way would take 5,000-10,000 years, the simulations showed. But once it had started, the discharge of ice would slowly but relentlessly continue until the whole basin was empty, even if the climate ceased to warm.

Unavoidable consequences

"This is the underlying issue here", said Matthias Mengel. "By emitting more and more greenhouse gases we might trigger responses now that we may not be able to stop in the future."

He told the Climate News Network: "While the sparse existing observations do not indicate warmer water inflow towards the Wilkes ice sheet margin at present, there is no reason why changes similar to those in West Antarctica could not also occur here."

The possibility remains a distant prospect. Mengel said some simulations produced the warm ocean conditions needed to remove the ice cork within the next 200 years, but It would take around 2,000 years to raise global sea levels by one metre.

He added: "The issue is that this would then be unstoppable ... We have detected a new and previously overlooked source of sea level rise, therefore these numbers have to be added to the present sea level rise projections of the IPCC.

"Sea level as projected for the forthcoming centuries is already potentially devastating for many coastal areas around the globe. Every centimetre of sea level rise on top of these projections is even more significant."